Most living things exhibit striking seasonal changes during the course of the year. In many cases, the changes are known to be induced by seasonal changes in daylength, or photoperiod. Changes in the photoperiod generally affect animals in two ways. First, the constant daily expansion or contraction of the photoperiod can cause an animal's daily activity or rest phase to expand or contract, so that it matches the photoperiod. Second, specific changes in the photoperiod can trigger new behaviors, such as hibernation, migration, or breeding, that represent adaptations to conditions that prevail during a particular season. These new behaviors are often accompanied by and depend on marked changes in the biochemistry, physiology and anatomy of the organism. Basic researchers have proposed that the photoperiod controls the organism through its effects on a biological clock mechanism that employs two separate circadian oscillators to track the constantly changing times of dawn and dusk through the course of the year. This project is designed to investigate whether human biology is similarly subject to control by changes in the photoperiod. We monitor daily patterns of sleep, hormones and body temperature in individuals who live for one month on a schedule of light exposures that simulate a natural winter day. They move about freely in ambient natural and artificial light for ten hours each day but are confined to a dark room for fourteen hours each night. For comparison, we make similar measurements in the same individuals while they live for one week on a conventional schedule in which they are exposed to ambient light for sixteen hours each day and sleep in a dark room for eight hours each night. The results of the experiment suggest that the human circadian system is similar to that of other animals-that it is composed of two subsystems, one of which is synchronized with dawn and the other with dusk. This conclusion is based on the following observations: After night was artificially expanded to fourteen hours, sleep divided into two components that gradually moved apart. The evening component (E) was associated with the daily offset of cortisol secretion, onset of melatonin secretion, onset of sleep, peak of slow wave sleep and decline in body temperature, and it was entrained to dusk. The morning component (M) was associated with the daily onset of cortisol secretion, offset of melatonin secretion, peak of REM sleep, rise in body temperature and onset of wakefulness, and it was entrained to dawn. The results are likely to have important implications for human biology and medicine.